1. Field of the Invention
Embodiments described herein generally relate to processes for manufacturing maleic anhydride. Specifically, processes and solvents for recovering a crude maleic anhydride stream from a reactor effluent are described.
2. Description of the Related Art
Maleic anhydride (cis-butanedioic anhydride) is used alone or in combination with other acids in the manufacture of alkyd and polyester resins. It is also a versatile intermediate for chemical synthesis. Global demand for maleic anhydride in 2009 was approximately 1.7 MT (million metric tons), with demand growth estimated by some at about 4% through 2020.
Maleic anhydride is prepared commercially by contacting a feed gas comprising molecular oxygen and a suitable hydrocarbon (e.g., n-butane or butene) with a vanadium-phosphorus-oxygen catalyst to partially oxidize the hydrocarbon and produce maleic anhydride. Hydrocarbons are converted to maleic anhydride by passing the feed gas through a reactor containing a fixed or fluidized bed of catalyst. The reaction product gas which is produced contains maleic anhydride together with oxidation by-products such as CO, CO2, water vapor, acrylic and acetic acids and other by-products, along with inert gases present in air when air is used as the source of molecular oxygen.
The prior art discloses a number of methods of isolating and recovering maleic anhydride from the reaction product gas. For example, the maleic anhydride can be recovered by direct condensation from the reaction product gas or by scrubbing the gas with water and dehydrating the resulting aqueous mixture by azeotropic distillation with xylene. However, due to increased product yields, the preferred method of recovery comprises selectively absorbing the maleic anhydride in a suitable solvent and subsequently stripping the maleic anhydride from the resulting absorption liquor to obtain crude product.
U.S. Pat. No. 4,118,403 (White) discloses contacting the reaction product gas with an organic solvent in an absorber column so that the maleic anhydride, as well as some of the oxidation by-products, are absorbed in the solvent. The solvent comprises a dialkyl phthalate having 2 to 8 carbon atoms in each alkyl chain (e. g., dibutyl phthalate), and from about 0.5 to about 10 weight percent phthalic anhydride. U.S. Pat. No. 3,891,680 describes carrying out the washing operation in esters formed from phthalic acid with C4-C5 alcohols. According to German O.S.2,444,824, dibenzylbenzol is used as the wash fluid. British Pat. No. 1,443,411 describes polymethylbenzophenone as the wash fluid.
European Published Patent Application 0 459 543 A1 discloses a process for recovery of maleic anhydride from reaction mixtures resulting from the catalytic oxidation of butane, butene, mixtures of hydrocarbons containing four carbon atoms, or benzene in which the gaseous reaction mixture containing maleic anhydride is contacted with an organic solvent such as dibutyl phthalate to absorb maleic anhydride contained in the reaction mixture. The maleic anhydride-enriched solvent is contacted with a gas of low relative humidity at a pressure of between 0.01 and 2.0 bar and at a temperature above 80° C. to remove water and then continues to a separation step in which maleic anhydride is stripped from the solvent, whereupon the resulting lean solvent is washed with water to remove contaminants before being recycled to the adsorption zone.
European Patent EP 0 815 098 B1 claims that the solvent preferably comprises a dialkyl phthalate compound having from two to eight carbon atoms in each alkyl chain. Suitable dialkyl phthalate compounds include: dimethyl phthalate, diethyl phthalate, dipropyl phthalate, diisopropyl phthalate, dibutyl phthalate, diisobutyl phthalate, dimethyl dihydrophthalate, diethyl dihydrophthalate, dipropyl dihydrophthalate, diisopropyl dihydrophthalate, dibutyl dihydrophthalate, diisobutyl dihydrophthalate, dimethyl tetrahydrophthalate, diethyl tetrahydrophthalate, dipropyl tetrahydrophthalate, diisopropyl tetrahydrophthalate, dibutyl tetrahydrophthalate, and diisobutyl tetrahydrophthalate, dibutyl phthalate being the most preferred maleic anhydride-absorbing solvent.
Although extraction of maleic anhydride using phthalates as a solvent is widely taught and practiced today, the cost of phthalates such as dibutyl phthalate is increasing. Thus, there is a need for a lower cost maleic anhydride extraction process and solvent.